The present invention relates to a gasket material to be mounted on an engine of a vehicle, and particularly relates to a gasket material improved in durability against antifreeze liquid.
A rubber-coated stainless steel plate in which a rubber layer is laminated to a stainless steel plate is generally used for a gasket, particularly a head gasket to be mounted on an engine of a vehicle. In addition, as disclosed on Japanese Application Publication Number 03-227622 (JP3-227622A), a gasket material in which a chromate film made from a chromium compound, phosphoric acid and silica is formed on one or each of opposite surfaces of a stainless steel plate in order to retain a rubber layer more firmly, and the rubber layer is laminated onto the chromate film, is also used.
Considerable two main damages of a gasket material caused during an operation of an engine are followings:                1. A destruction of adhesion between the rubber layer and the metal plate caused by an infiltration of coolant water and/or lubrication oil.        2. A destruction of adhesion between the rubber layer and the metal plate caused by heat from combustion of the engine.        
In order to prevent these destructions, it is general to subject a chromate film between the rubber layer and the metal plate. The reason why the gasket material with the chromate film has high adhesiveness is because the adhesion between the chromate film and the metal plate and the adhesion between chromate film and the rubber layer are strong, and considered as follows:                The strong adhesion between the chromate film and the metal plate is achieved, because, when chromate treatment liquid is applied on the metal plate, a surface of the metal plate is etched by dichromate included in the liquid, polar components are generated on the surface, and then the polar components and the chromate film are strongly adhered via a secondary bond.        The strong adhesion between the chromate film and the rubber layer is achieved, because, polar bases of the heated-and-dried dichromate and the silica are strongly adhered to the polar bases of the rubber layer.        
Such a gasket material in which a rubber layer is provided on a stainless steel plate subjected to chromate treatment, is superior in heat resistance, adhesion with antifreeze liquid, or the like. However, with the rising environmental awareness in recent years, there is a tendency to avoid chromate treatment because the chromate treatment has a problem that hexavalent chrome contained in a chromate treatment solution has a damaging effect directly on the human body. In addition, there is also a problem that effluent containing hexavalent chrome has to be subjected to special treatment according to Clean Water Law, and waste of a stainless steel material subjected to chromate treatment cannot be recycled. Further, it is also highly likely that chrome in the chromate film is extracted due to contact with antifreeze liquid or oil. Thus, the gasket material subjected to chromate treatment has serious environmental problems.